Description of the Prior Art
FIG. 9 shows general arrangement of a press machine.
In this figure, the reference numeral 72 represents a crankshaft. On an eccentric portion 72e, a slide 74 is connected via a connecting rod 73. On the crankshaft 72, rotary motive power is applied from a driving shaft 71 via a gear train 75. A flywheel 76 rotated and driven by a motor 77 via a belt 78 is connected to or separated from the driving shaft 71 by a clutch device 80. On the other hand, the driving shaft 71 is braked by a brake device 90.
Therefore, when the brake device 90 is released (turned OFF) and the clutch device 80 is connected (turned ON), rotating energy accumulated on the flywheel 76 is transmitted to the driving shaft 71, and this makes it possible to move a slide 74 up or down via the gear train 75, the crankshaft 72, and the connecting rod 73. To stop the slide 74 at a desired stop position such as top dead center, it is widely known that the clutch device 80 should be separated (OFF) and the brake device 90 should be operated (ON).
The clutch device 80 is turned on when a solenoid 15 is energized (turned ON) to open a solenoid valve 15V and air under a predetermined pressure P is supplied to a supply inlet through an air feed piping 81. The clutch device is turned off when the solenoid 15 is de-energized (turned off) by resilient force of a spring when air pressure is released. On the other hand, the brake device 90 is turned on by resilient force of a spring in normal condition, and it is turned off when the solenoid 15 is energized (turned on) to open the solenoid valve 15 and the air under a predetermined pressure P is supplied through an air feed pipe 91. Accordingly, it is generally arranged that the two solenoids 15 and 15 are energized (turned ON) at the same time.
Description will be given later on 1P, 30P and 10P, which constitute a drive control in FIG. 9. In FIG. 9, a separate type arrangement is shown, while it is essentially the same as a combination type clutch-brake (80, 90), in which the clutch device 80 and the brake device 90 are integrated together.
Meanwhile, the driving control of the solenoid 15 for solenoid valve, which supplies and discharges the air for the clutch-brake (80, 90) of a press machine, must be performed timely, quickly and accurately.
In a press machine, which is operated at more than 400 SPM, the follow-up speed cannot catch up if it is controlled by a drive control comprising a mechanical switch, an auxiliary relay, etc., and a semiconductor device must be adopted as a power switch.
For this reason, the following arrangement has been used in the past: As shown in FIG. 10, a drive control signal CNT is generated according to an ON-OFF command signal (running signal) of the solenoid 15, and either one of solenoid driving circuits (10P, 110P) is selectively adopted. That is, either an AC power system (10P), in which an AC switching semiconductor element (such as TRIAC) under ON-OFF control by the above drive control signal CNT (trigger pulse TP) and a solenoid 15 are connected in series with an AC power supply (AC), or a DC power system (110P), which comprises a rectifier (Ref) with its primary side connected in series with an AC power supply (AC) as shown in FIG. 11 and a DC switching semiconductor element 111 (such as transistor) connected in series with a solenoid 115 on a secondary (DC) side of the rectifier (Ref) has been selectively adopted.
Generally, the DC power system (110P) of FIG. 11 can turn on and off the solenoid 115 with no variation in time to ON-OFF of the driving signal DRV, but it is disadvantageous in that the OFF time may be extended depending upon constant of electric circuit such as a diode for surge absorption. On the other hand, it is possible in the AC power system (10P) of FIG. 10 to drive at high speed and at low cost because of power facility. Thus, the AC power system is adopted in most cases.
To ensure higher safety, it is attempted to constitute the solenoid valve by a double solenoid valve comprising two solenoids 15 and 15 and to connect the two solenoids 15 and 15 in parallel to a semiconductor element 11 shown in FIG. 10, or to provide a respective solenoid driving circuit (10P, 10P) for each solenoid 15.
A conventional type drive control of an AC power system comprises, as shown in FIG. 10, a running operation panel (1P) having a manual running button 2 generally connected to a DC power supply (DC), a control panel 30P including drive control signal generating means for outputting the drive control signal CNT according to an ON-OFF command signal (running signal) issued from the running operation panel (1P) when the manual running button 2 (auxiliary relay contact if such is used) is turned on, and said solenoid driving circuit 10P.
In case a drive control of an AC power system is adopted for a clutch-brake of a press machine, special care is taken on components to ensure more safety. In particular, in the semiconductor element 11 of FIG. 10, which forms an essential part, a fault is likely to occur symmetrically, i.e. a fault is likely to occur on continuity side or on cut-off side. Even when the drive control signal CNT (TP) is turned on, the solenoid 15 is not driven if a fault occurs on the cut-off side of the semiconductor element 11. However, such a condition is on the so-called safety side, and this does not become an issue very often.
In contrast, even when the drive control signal CNT is turned off, it is very dangerous, if the semiconductor element 11 is in trouble on the continuity side. Because the press is not stopped even when a press command signal is issued (running signal S and drive control signal CNT are turned off), there may occur problems such as damage of equipment including dies or a serious human accident.
That is, even when a stop command has been given, it is absolutely not allowed that the press machine of FIG. 9 (slide 74) is continuously operated (upward and downward reciprocal movement), because this will impair safety and reliability of the system.
Further, even when the solenoid driving circuit 10P including the semiconductor element 11 is normal, the safety and the reliability of the entire system are impaired if the drive control signal generating means (30P) and the running operation panel 1P are in trouble. That is, the running signal S and the drive control signal CNT must be truly effective because, even when these two signals S and CNT are abnormal signals, the solenoid driving circuit 10P turns the solenoid (clutch-brake) on. Particularly, it causes a serious problem if the AC power system (AC) is adopted for the running operation panel 1P for the same reason in the case of the solenoid driving circuit 10P.
Thus, in case a drive control of the AC power system using the semiconductor element 11 is adopted in a clutch-brake of a press machine, it is important not only to increase the reliability of the semiconductor element and circuit system, but also to provide reliable and fail-safe operations to ensure safe and smooth operation of the system.